Glaucoma is a term describing a group of eye disorders caused when the intraocular pressure within the eye increases, thereby causing retinal and optic nerve damage and subsequent loss of vision. The anterior chamber is the cavity located between the cornea and the lens, and is filled with a fluid (i.e., aqueous). Aqueous fluid is continuously produced by secretions from the ciliary body. The aqueous drains from the anterior chamber through the drainage angle and into the venous system. In a normal situation, aqueous production is equal to aqueous outflow through the drainage angle (angle), and intraocular pressure remains fairly constant in a range considered to be safe, for example, 15 to 21 mmHg range. Glaucoma occurs when aqueous does not drain sufficiently from the anterior chamber through the angle, causing an increase in intraocular pressure above the safe range. Raised intraocular pressure (generally above 21 mm Hg in humans and above 28 mm Hg in cats, dogs, and horses) is the most important and currently the only modifiable risk factor for treating glaucoma. Lowering this intraocular pressure is the major treatment goal in all glaucoma patients.
In the human eye, aqueous drains from the anterior chamber through the trabecular meshwork into a collecting channel, called Schlemm's canal. From Schlemm's canal, aqueous flows into collector channels that join Schlemm's canal, and then into the episcleral venous system. However, the anatomy of the canine, feline, and equine iridocorneal drainage angle has significant differences compared with the human eye. These eyes have pillars of tissue (pectinate ligaments) as the most anterior part of the iridocorneal angle, which communicate with a wide region (the ciliary cleft) that drains aqueous into the uveal and corneoscleral trabecular meshwork. From there, aqueous enters into one or more drainage veins that comprise the angular aqueous plexus (AAP), and then exits the eye via episcleral veins.
In all species, glaucoma can be roughly divided into two main categories, “open-angle” (or OAG) and “closed angle” or “angle-closure” (or ACG) glaucoma. In open-angle glaucoma, the anterior chamber remains open, but the exit of aqueous through the trabecular meshwork is reduced. Closed-angle glaucoma is caused by closure of the angle, preventing drainage of aqueous out of the anterior chamber.
Glaucoma can also be classified as “primary” (inherited) and “secondary” (non-inherited). Secondary glaucoma can be caused by injury, abnormal structures, inflammation, tumors, certain drugs, or diseases. Both primary and secondary glaucoma can be open or closed angle.
Despite its importance, the long-term control of human and veterinary glaucoma continues to be a challenge and is often unsuccessful in controlling the glaucoma and/or maintaining vision. Current treatments for glaucoma include medications and/or surgery to decrease intraocular fluid production, increase fluid drainage from the eye, or both.
In both human and veterinary medicine, few advancements have been made in medical and surgical therapy for glaucoma. Medical and surgical treatment is expensive and unaffordable to a large majority of human and veterinary glaucoma patients. Medications can cause side effects and are frequently ineffective in long-term control of glaucoma. When drug therapy fails, surgical therapy is needed.
Various forms of surgery to treat glaucoma include methods to open the fluid drainage channels, reduce fluid production by the ciliary body, or both. Multiple surgeries may be required, and frequently medications are also needed to help control intraocular pressure postoperatively.
In humans, there are many different surgical procedures for control of open-angle glaucoma. These surgeries involve procedures that mechanically disrupt the trabecular meshwork, improve outflow of aqueous through the drainage angle, making holes in the peripheral iris, filtering procedures (penetrating or non-penetrating), tube shunts (valved or non-valved), reduce aqueous fluid production, or involve manipulations of Schlemm's canal These surgeries are all major operations that are designed for treatment of open-angle glaucoma, Most of these surgical techniques are not applicable for the canine, feline, or equine eye because of the lack of Schlemm's canal in these species, because they are designed mainly for the treatment of open-angle glaucoma rather than closed-angle glaucoma.
Various tube-shunt drainage devices have been developed for treatment of closed-angle glaucoma and divert aqueous fluid out of the eye to the subconjunctival space (Ahmed valve, Molteno glaucoma shunt) or into the frontal sinus or nasal cavity (Cullen shunt) Shunt surgeries eventually fail due to clogging of the drainage tube, and/or due to formation of a fibrous tissue capsule over the shunt, causing decreased flow of aqueous fluid out of the eye. This causes return of glaucoma and necessitates further treatment.
All of the above surgeries have numerous disadvantages including poor long-term prognosis for control of glaucoma. They need to be performed in an operating room, involve substantial trauma to the eye, require great surgical skill, have the potential for significant complications, are expensive, and have limited to no availability for many pet owners. Most significantly, the anatomy of the canine, feline, and equine iridocorneal filtration angle has significant differences compared with the human eye, which renders most human glaucoma-management surgeries ineffective in dogs, cats, and/or horses
Acute angle-closure glaucoma in dogs is an emergency and requires that the intraocular pressure be reduced to a safe range within minutes or hours. Glaucoma surgeries which attempt to preserve vision are often declined by clients because they may need to travel long distances to find a veterinary ophthalmologist that can provide such surgeries, the cost of surgery is very expensive, pre- and post-surgical treatments are time-consuming and expensive, and the patient may be uncooperative. If treatment is delayed, permanent blindness will usually occur, often within a matter of a few hours. In most dogs, glaucoma is also a very painful condition. Once the eye is blinded by glaucoma, then glaucoma-eliminating surgery (such as enucleation, i.e., removal, of the eye, evisceration of the eye with surgical placement of an intraocular prosthesis, or an ablation procedure) is required to restore comfort and eliminate the requirement for treatment. Therefore, new surgical approaches need to be developed that provide faster, better, safer, and less expensive care for both human and veterinary glaucoma patients, both in the short- and long-term.